feat: module-owned i18n with merge-at-boot translation loading

Each module can now ship its own i18n/ directory with translation files
(default_de.json, default_en.json). At boot time, ModuleI18nLoader
preloads core translations then merges module translations on top via
Reflection into I18n::$strings — the t() helper stays unchanged.

- Add i18nPath property to ModuleManifest
- Add ModuleI18nLoader with Reflection-based preload
- Extract 59 module-exclusive keys from core i18n into module files
  (notifications: 8, bookmarks: 41, addressbook: 10)
- Expand TranslationKeysTest to scan modules/ and validate against
  merged core + module translations
- Add ModuleI18nContractTest for per-module de/en parity and completeness

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/App/Module/ModuleI18nLoader.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace MintyPHP\App\Module;
+
+use MintyPHP\I18n;
+use ReflectionProperty;
+
+/**
+ * Preloads core + module translations into I18n::$strings at boot time.
+ *
+ * Since I18n::$strings is private static and lives in a vendor class,
+ * Reflection is used once per locale to inject the merged translation map.
+ */
+final class ModuleI18nLoader
+{
+    /**
+     * @param list<ModuleManifest> $manifests Active module manifests (already sorted)
+     * @param string $coreI18nDir Absolute path to the core i18n/ directory
+     */
+    public static function preload(array $manifests, string $coreI18nDir): void
+    {
+        $locales = ['de', 'en'];
+        $domain = I18n::$domain ?: 'default';
+
+        $prop = new ReflectionProperty(I18n::class, 'strings');
+
+        foreach ($locales as $locale) {
+            // Load core file
+            $coreFile = $coreI18nDir . '/' . $domain . '_' . $locale . '.json';
+            $merged = is_file($coreFile)
+                ? (json_decode(file_get_contents($coreFile), true) ?: [])
+                : [];
+
+            // Merge each module's translations on top
+            foreach ($manifests as $manifest) {
+                if ($manifest->i18nPath === null) {
+                    continue;
+                }
+                $moduleFile = $manifest->i18nPath . '/' . $domain . '_' . $locale . '.json';
+                if (!is_file($moduleFile)) {
+                    continue;
+                }
+                $moduleStrings = json_decode(file_get_contents($moduleFile), true);
+                if (is_array($moduleStrings)) {
+                    $merged = array_merge($merged, $moduleStrings);
+                }
+            }
+
+            // Inject via Reflection into I18n::$strings
+            $strings = $prop->getValue();
+            $strings[$domain][$locale] = $merged;
+            $prop->setValue(null, $strings);
+        }
+    }
+}